Silanized Aryl Layers through Thiol-yne Photo-click Reaction

Article pubs.acs.org/Langmuir

Silanized Aryl Layers through Thiol-yne Photo-click Reaction Marwen Bengamra,† Akila Khlifi,† Nadia Ktari,† Samia Mahouche-Chergui,‡ Benjamin Carbonnier,‡ Najla Fourati,§ Rafik Kalfat,*,† and Mohamed M. Chehimi*,‡,⊥ †

Laboratoire Méthodes et Techniques d’Analyse, Institut National de Recherche et d’Analyse Physico-Chimique, BiotechPole Sidi-Thabet, 2020 Ariana, Tunisia ‡ Institut de Chimie et des Matériaux Paris Est − Equipe Systèmes Polymères Complexes, UMR 7182 CNRS, Université Paris Est Créteil, 2-8 rue Henri Dunant, 94320 Thiais, France § SATIE, UMR CNRS 8029, ENS Cachan − Cnam, Cnam, 292 rue Saint Martin, 75003 Paris, France ⊥ ITODYS, UMR CNRS 7086, Université Paris Diderot, Sorbonne Paris Cité, 15 rue J-A de Baïf, 75013 Paris, France ABSTRACT: Nanometer-scale multilayered coatings were prepared by sequential surface reactions on gold plates. First 4-ethynylphenyl organic layer was electrografted from the parent diazonium tetrafluoroborate salt providing reactive alkynylated gold plate (Au-Y). The latter served for clicking mercaptosilane via a thiol-yne photo-triggered reaction to obtain alkoxysilanefunctionalized surface. The trialkoxysilane top groups in turn served as anchor sites for the final sol−gel coating resulting from the surface reaction between aminopropylsilane and tetraethoxysilane (TEOS). It is demonstrated that two coupling agents, namely, aryl diazonium salt and silane, can be coupled using photo-triggered thiol-yne click reaction, resulting in robust multilayered coatings. In addition, the process is versatile in that it offers the possibility to design patterned surfaces. The top sol−gel layer can in turn be reacted with aminosilane, therefore providing a reactive and functional surface that can be used for different applications given the reactivity of amine groups. This approach opens new avenues for photo-triggered click reactions of aryl layers from diazonium salts. It shows that the new class of surface modifiers and coupling agents has much to offer and continues to be renewed for achieving tightly bound, reactive top coatings.

1. INTRODUCTION Sol−gel processing is a versatile technology for obtaining hydrophobic, scratch- and abrasion-resistant, corrosion-resistant, antibacterial, antireflective multifunctional coatings on different substrates.1 Because sol−gel technology uses a relatively low temperature during the formation of the inorganic matrix, various organic, inorganic, and biological molecules can be introduced as dopant agents without degradation risk.2 Moreover, sol−gel materials such as amorphous silica are nontoxic and biodegradable; hence, they can be utilized as biocarriers.3 The utilization of organofunctional silanes as precursors in sol−gel processes leads to the formation of organically modified hybrids that improve the architecture, flexibility, and density of the films and, additionally, provides functionality to the coatings. Additional important advantages of sol−gel processing are its cost-effectiveness and environmental safety.4 In adhesion science and technology, organosilanes are the most known coupling agents; they were made well-known by the late Edwin Plueddemann.5 They provide anchoring sites for polymers6 or polymerization initiators,7 sol−gel/nanoparticle hybrid coatings,8 proteins,9 and particles for optical analytical purposes,10,11 among other species. However, their chemistry is © 2015 American Chemical Society

often restricted to ceramics and clays and rarely applied to noble metals, unless mercaptosilanes are used because they interact strongly with gold or platinum surfaces via the thiol groups.12 The interaction of thiols with gold has indeed been the subject of numerous reviews.13−15 Unfortunately, thiols are known to have weak adhesion, particularly when they are heated to 70 °C.16 As an alternative to Au−S interfacial bonds one can envisage the formation of robust Au−C bonds using alkynes17 or diazonium salts.18 Whereas alkynes hold promise for surface modification, despite their use under an oxygen-free environment, the surface chemistry of diazonium salts was widely studied to construct excellent adhesive layers, under less restrictive conditions, that can withstand sonication and harsh treatments.19 They have shown superior adhesion over thiol self-assembled monolayers with comparable chemical structures.20 This work confirmed the findings of Gooding’s group, who reported that diazonium salts electrochemically grafted to a gold surface were more stable than chemisorbed alkanethiols with regard to long-term storage.21 Received: July 10, 2015 Revised: September 14, 2015 Published: September 16, 2015 10717

DOI: 10.1021/acs.langmuir.5b02540 Langmuir 2015, 31, 10717−10724

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Figure 1. Strategy for coupling aryl and silane layers through thiol-yne photo-click reaction. Au, pristine gold plate; Au-Y, alkynylated gold plates prepared by electroreduction of 4-ethynylbenzenediazonium salt; Au-Y-M, Au-Y clicked to MPTMS via photo-assisted thiol-yne reaction using Darocure photosensitizer and Au-Y-M-SG.

Besides the widely investigated Huisgen 1,3-cycloaddition,35 the 4+2 Diels−Alder cycloaddition together with the thiyl radical mediated addition of thiols onto unsaturated carbon− carbon bonds raised huge interest over the past few years.36 These addition reactions are tolerant to many functional groups and can be conducted under mild conditions. Moreover, the thiol-ene/yne variant does not involve toxic transition-metal catalysts. The versatility of the reaction is further enhanced by the fact that the formation of thiyl radicals can be easily initiated under either thermal or UV conditions with or without the aid of hydrogen abstractor.37,38 Restricting the discussion to surface functionalization, thiol-ene/yne click reactions have been applied successfully for a number of efficient surface coupling reactions.37,39−41 Of relevance to the actual work, photo-triggered thiol-yne reaction is one of the most investigated click strategies for making new materials.42 Particularly, thiol-yne photo-click is a metal-free reaction that can be confined to surfaces43 for the development of, for example, biosensors,44 polymer coatings with controlled friction,45 and dye-functionalized surfaces,46 to name but a few. However, despite these remarkable advances, diazonium salt-modified materials have never been subjected to surface-confined photo-triggered thiol-yne reactions, hence the motivation for this work. Herein, we explore the photocatalyzed thiol-yne coupling reaction in order to bridge two coupling agents of significantly distinct structures: 4-ethynylphenyl layers from the electroreduction of the diazonium salt precursor and mercaptosilane. The latter offers free Si−OR groups that further serve for in situ grafting of sol−gel top layers from the reaction of aminosilane with tetraethoxysilane (TEOS).

The recent years have witnessed important developments with the versatile surface chemistry of aryl diazonium salts as they efficiently modify a large panel of substrates through a plethora of approaches.18,22−25 The academic research achievements paved the way for industrial developments pertaining to specialty carbon blacks and drug-eluting stents.26 One key feature of diazonium salts is that they act as true coupling agents for macromolecules and particles owing to the high surface−aryl energy bonding27 which ensures the construction of robust interfaces/interphases in multicomponent materials. To prepare a sol−gel top layer, tightly bound to gold, we reasoned it would be possible to link a mercaptosilane to the noble metal through an alkyne-containing aryl primer layer. Herein we are facing an interesting surface chemistry problem: coupling two coupling agents, namely, silanes and diazonium salts. The concept has been approached by Shimura and Aramaki 28 in their study of ultrathin two-dimensional protective coatings of aryl and silane on iron. Recently, Bagheri et al.29 proposed to modify the inner surface of polymeric and metallic capillaries by sol−gel-based sorbent grafted to an aryl primer layer, therefore providing an unbreakable capillary microextraction column. Despite the interests and applicability of these methods in their respective domains (corrosion and separation science, respectively), they are time-consuming and require high operation temperature. A modern and efficient approach to address the construction of multilayered coatings would be click chemistry. There are many well-known click reactions30 that can be envisaged for building new polymers,31 grafting reactive and functional polymers,32 immobilizing nanoparticles,33 and assembling nanostructures.34 10718


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shows a broad peak starting from −0.3 V, in line with previously reported observations.50 After the first cycle, the current vanishes until the 30th cycle due to passivation by aryl multilayers. It is noted that in the actual work, electrografting was performed on gold electrodes for which Benedetto et al.51 reported multipeaks for a single diazonium compound and which accounts for grafting on distinct crystallographic sites of gold surface. It is thus likely that the broad voltammogram displayed in Figure 2 originated from the convolution of broad (due to the alkynylated diazonium salt), multiple electroreduction peaks (due to gold). After electrografting, the gold electrodes were thoroughly cleaned ultrasonically in ACN for 3 min, then in ethanol, and subsequently dried in a stream of argon. 2.3. Thiol-yne Click Reaction on Gold Surface. Gold substrates electrografted with 4-alkynylphenyl groups were reacted with MPTS (0.2 g at 50 mmol/L concentration) in 20 mL of anhydrous toluene. The MPTS solution was degassed with argon to remove oxygen, therefore preventing inhibition of the radical-mediated click grafting process. The click grafting was achieved by irradiating UV light (365 nm, power density = 5 mW/cm2) at room temperature for 1−3 h using a Spectrolinker XL 1500 UV in the presence of 2-hydroxy-2methyl-1-phenylpropanone (16 mg) as a photo-initiator. After the reaction, the electrodes were thoroughly washed with toluene and acetone to remove unreacted silane and finally dried under argon. 2.4. Preparation of Sol−Gel Top Layers. The sol−gel top layers were prepared on modified gold electrodes by polycondensation between the surface clicked MPTS and APTMS in the presence of TEOS used as a cross-linker. First, the gold electrodes were dipped in 5 mLof a methanol solution of TEOS (436 μL, 410 mg) taken as porogenic solvent and were shaken for 2 h before the addition of APTMS (192 μL, 180 mg). The reaction was left to proceed, under shaking using a Specimix apparatus, during 24 h at room temperature. Finally, the gold plates were sonicated in 5 mL of methanol for 5 min and then washed with ethanol to remove organic molecules. The sol− gel-coated electrodes were dried and stored under argon. It is worth noting that at this stage the sol−gel process was confined to the surface only and that no inorganic polymer of the sol−gel type was formed in solution. 2.5. Characterization. XPS spectra were recorded using a Thermo VG Scientific ESCALAB 250 system fitted with a microfocused, monochromatic Al Kα X-ray beam (1486.6 eV, 500 μm spot size). The spectra were calibrated against the Au 4f7/2 peak set at 84 eV. Other experimental details such as specimen mounting, acquisition parameters, and quantification are similar to those previously reported.52 AFM images of untreated and modified gold samples were obtained with an XE-70 atomic force microscope in the tapping mode using a Si3N4 tip cantilever. The cantilever oscillation frequency was set at 267 Hz. ATR spectra were recorded using an EQUINOX 55 (Bruker) spectrometer. Spectra were accumulated 64 times at 4 cm−1 resolution and processed using OPUS/IR software. 2.6. Electrochemical Blocking Property Measurement. The blocking of electrochemical activity of bare and modified surfaces was studied by cyclic voltammetry using 1.0 mM ferrocene/ferrocinium (Fc/Fc+) redox couple in 0.1 M TBAT, in 20 mL of ACN. The experiments were conducted with a three-electrode system having a stainless steel mesh as counter electrode, saturated calomel (SCE) as reference electrode, and gold as working electrode. The cyclic voltammograms were recorded from −0.1 to 1.0 V with a scan rate of 50 mV/s for one cycle using Biologic potentiostat (model SP-150).

Figure 1 displays the three main steps to obtain sol−gel layers: (i) synthesis of 4-ethenylbenzenediazonium salt and electrografting of the corresponding 4-alkynylphenyl groups, (ii) photo-triggered click coupling reaction between alkynyl and thiol groups, and (iii) polycondensation of APTMS and TEOS to obtain sol−gel top layers. The physicochemical attributes of the layered coatings were characterized by XPS, attenuated total reflection (ATR), and AFM. The blocking properties of the multilayered coating were probed electrochemically using a ferrocene/ferrocenium (Fc/ Fc+) redox couple.

2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. Gold-coated glass slides (5 ×15 mm) with a 55 nm thick gold coating were purchased from SSens, The Netherlands. Prior to their use, gold electrodes were rinsed by sonication in acetonitrile (ACN), water, and ethanol to remove the organic residues and then dried in a stream of argon. In the second stage, the plates were cleaned using the UV/ozone (Boekel, Inc., model 135 500) as described previously47 and immediately dipped in acetonitrile for electrografting experiments. 4-Ethynylaniline, 3-mercaptopropyltrimethoxysilane (MPTS), and 3-aminopropyltrimethoxysilane (APTMS) (Sigma-Aldrich); tetrabutylammonium tetrafluoroborate, TBAT (Fluka); and tetraethoxysilane, TEOS (Acros Organics), were used as received. pH 7.4 aqueous buffer solution was prepared by dissolving 2.85 g of NaH2PO4 and 5.60 g of Na2HPO4 in 1 L of deionized water.48 Water was deionized using a Millipore purification system. 2.2. Synthesis and Electrografting of the Diazonium Salt [HCCC6H4N2]BF4. The diazonium salt 4-ethynylbenzenediazonium tetrafluoroborate [HCCC6H4N2]BF4 was synthesized from the commercially available aromatic amine: 4ethynylaniline (1 g, 8.5 × 10−3 mol) was dissolved in 100 mL of acetone and an aqueous solution of HBF4 (1.5 g, 1.7 × 10−2 mol) and the mixture was maintained under stirring at 0 °C. NaNO2 (1.75 g, 1.7 × 10−2 mol) was added to the solution under vigorous stirring, and the reaction was left to proceed for 30 min. The reaction mixture was kept in the freezer at 0 °C for 1 h to precipitate the diazonium salt. The latter was then filtered and rinsed with a copious amount of cold diethyl ether. The salt was then dried under vacuum and kept at −10 °C. Gold substrates were modified by the electrochemical reduction of the diazonium salt using cyclic voltammetry.49 Electrografting was carried out in a degassed solution of 5.0 mM of the diazonium salt and 0.1 M supporting electrolyte (TBAT) in acetonitrile. The potential was scanned 30 times between 0 and −1.0 V versus SCE. Figure 2

3. RESULTS AND DISCUSSION 3.1. Construction and Properties of Multilayers. This work describes the formation of multilayered coatings on gold platforms using a combination of surface chemistry techniques: (i) electroreduction to tether photo-clickable alkynylated aryl layers; (ii) photo-induced click reaction between alkynyl groups from the aryl layers and the thiol from the mercaptosilane;and

Figure 2. Electrografting of 4-ethynylphenyl groups in ACN by 30 consecutive cycles. Scan rate = 50 mV S−1. 10719


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Langmuir (iii) finally, a sol−gel chemical route for the growth of a polysiloxane top layer. It is noteworthy that the photo-click reaction step can advantageously be explored in view of making patterned surfaces. XPS and ATR were employed to determine the surface compositions of the layered coatings. The blocking properties were investigated by cyclic voltammetry, and patterned surfaces were characterized by XPS and AFM. 3.2. ATR Analysis. The chemical composition changes at the surface of gold plates were further investigated by infrared spectroscopy. Figure 3 displays ATR spectra of Au-Y, Au-Y-M,

the grafting of the APTMS by the sol−gel process, there is a striking difference between the spectra of Au-Y-M and Au-Y-MSG: (i) Au-Y-M-SG exhibits a doublet at 1554 and 1626 cm−1 assigned to the NH2 deformation, and (ii) a broad band at 3050−3418 cm−1 assigned to the axial stretching of OH groups resulting from noncondensed silanols. In this particular spectral domain, there is a contribution of the NH stretching in the amine groups from APTMS. One can also note that the band centered at 1117 cm−1 becomes quite broad after attachment of the top sol−gel layer with a component at ∼1020 cm−1 assigned to the SiOSi vibrations. 3.3. AFM Imaging. The changes of the surface morphology were further examined by AFM (Figure 4). The modified electrodes show features that are not present on the smooth, bare gold. Interestingly, the film topography is closely related to its chemical composition. AFM images show that the entire gold plate is covered by the aryl layer, although the coating is patchy (Figure 4a). After the photoinduced thiol-yne reaction, a rather uniform film is visible with an average roughness of 12 nm (Figure 4b). One can also note that the sol−gel process results in the uniform growth of the film, and the roughness increased from 12 to 120 nm, resulting in a total covering of the gold electrode surface (Figure 4c). 3.4. XPS Analysis. Figure 5 displays the survey regions of Au, Au-Y, Au-Y-M, and Au-Y-M-SG. The main peaks Au 4f7/2, Si 2p, S 2p, C 1s, and O 1s are centered at 84, 103, 164, 285, and 533 eV, respectively. The electrografting of the alkynylated aryl layer (Figure 5a) induces a dramatic change in the spectral background of gold plate compared to the spectrum of the bare gold (Figure 5b), in that the gold substrate experiences important inelastic emission of photoelectrons that is reflected in the concave shape of the baseline. The C 1s/Au 4f intensity ratio increases owing to the aryl layer. After clicking the MPTS to Au-Y, one can clearly observe the appearance of both Si 2p and S 2p and an increase in the relative peak intensity of O 1s (Figure 5c) due to the clicked mercaptosilane layer. The latter screens the underlying alkynyl layer as evidenced by an increase in the inelastic background at the high binding energy side (low kinetic energy side) of the C 1s region (Figure 5c). The only persistent gold core level peaks are Au 4f and Au 4d doublets. In contrast, the Au 4p and Au 4s features are completely screened, implying that the thickness of the whole coatings exceeds the analysis depth of Au 4d, which is about 8 nm. Attachment of the mercaptosilane layer provides a platform for the growth of the sol−gel top layer. The process yields a layer that is so thick that even inelastically scattered gold

Figure 3. ATR spectra of Au-Y, Au-Y-M, and Au-Y-M-SG.

and Au-Y-M-SG. The presence of the 4-alkynylphenyl groups on the surface is confirmed by the presence of the aromatic CC at 1600−1450 cm−1 and CC at 2100 cm−1. The peak at 3250 cm−1 is characteristic of the CH stretching vibration in a terminal CH alkyne group. The mercaptosilane film gives a sharp SiO stretching band at 1117 cm−1, whereas the band centered at 830 cm−1 is attributed to SiC stretching vibration. The peak at 1459 cm−1 is assigned to the CH2 deformation, and the convoluted peaks centered at 2936 and 2845 cm−1 are due to the CH2 and CH3 stretching vibrations. The successful click attachment of the MPTS to the surface is confirmed by the disappearance of the band −CC− (expected at 2100 cm−1) and the appearance of a new band at 695 cm−1 corresponding to SC stretching vibration. After

Figure 4. Comparison of AFM images obtained for (a) Au-Y, (b) Au-Y-M, and (c) Au-Y-M-SG. 10720


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Figure 5. XPS survey regions from (a) Au-Y, (b) Au, (c) Au-Y-M 2h, and (d) Au-Y-M-SG. For the sake of clarity we plotted the Au survey region (b) below that of Au-Y (a).

marker, could provide useful insights into the electron-transfer barrier properties of the different grafted layers on gold electrodes. Cyclic voltammograms of 5.0 mM ferrocene shown in Figure 6 were obtained at (a) bare gold electrode, (b) Au-Y,

photoelectrons are quasi no longer observed. For this reason, the background is flat without any sign of elastic or inelastic Au photoelectrons (see survey scan of Au-Y-M-SG in Figure 5d). Detection of gold in this case was possible only after several accumulations of the noisy Au 4f doublet. The thickness of the layers was estimated by peak shape analysis using the Tougaard’s QUASES software (QUantitative Analysis of Surfaces by Electron Spectroscopy)53 and found to be ∼5, 12, and 20 nm for the aryl layer, the clicked mercaptosilane intermediate layer, and the top-sol gel (2h) coating. The XPS-determined surface elemental compositions of the gold plates are reported in Table 1. Table 1. XPS-Determined Surface Chemical Composition of Pristine and Modified Gold Plates

a

material

Au

Au Au-Y Au-Y-M 1h Au-Y-M 2h Au-Y-M 3h A-Y-M-SG

58.5 7.6 0.70 0.25 0.20 0.00

Si

8.30 13.0 13.7 19.4

S

C

N

O

9.60 9.1 9.2 0.34

33.5 74.3 62.6 54.5 53.3 46.3

2.3 2.0 tra tr 6.1

8.0 15.8 16.8 23.2 23.6 27.8

Figure 6. Cyclic voltammograms of 5.0 mM ferrocene at (a) bare gold electrode, (b) Au-Y, and (c) Au-Y-M-SG. Scan rate = 50 mV s−1.

tr, traces.

and (c) Au-Y-M-SG electrodes, respectively. As a consequence of film formation on Au, the intensity of the well-defined peaks observed at the bare gold electrode sharply decreased due to the presence of the aryl layer. However, still a low response of the Fc/Fc+ is visible, which is in line with the literature.54 Upon top-coating of the electrode by the sol−gel layer, a flat response was recorded with the Fc/Fc+, implying the efficient screening of the electrode (see also XPS below). Moreover, it is noteworthy that the voltammetric response turned from peak-shaped signals to waves of sigmoidal form, suggesting the predominance of radial diffusion instead of the classical linear mode of mass transfer. This was due to the presence of a physical barrier limiting somewhat the probe diffusion through the film from the solution to the electrode. 3.6. Patterned Surfaces. As a demonstration of proof of concept, we shall take advantage of the photo-triggered thiolyne reaction to pattern gold plates by sol−gel stripes. To make the stripes, a Teflon mask has been fabricated by cutting 1.0 mm wide rectangular windows. The mask was held on the electrode sample pretreated with an aryl layer. The thiol-yne

Au at. % sharply decreases after aryl grafting and then vanishes quasi completely. Carbon increases due to the aryl layer but then slightly decreases due to the attachment of the silane layers. The C/Si ratio is slightly higher than 3 for Au-YM plates, indicating that the aryl layer is still detected through the mercaptosilane. However, the 1 h click reaction is not sufficient to screen the aryl layer. For this reason it is necessary to continue the photo-induced reaction until at least 2 h to reach a steady state. The (O + Si)/S atomic ratio for the Au-YM series is about 4.4 for 2 and 3 h photo-triggered click reaction, which accounts for the chemical structure of the silane. The nitrogen peak is due to NN− azo groups within the aryl layer; its content decreases after 1 h of thiol-yne grafting due to the screening of the aryl layer, vanishes for 2 and 3 h click reaction, but then increases to 6.1% after making the TEOS-APTS sol−gel top coat. 3.5. Electrochemical Blocking Property Study. Cyclic voltammetric blocking experiments, using ferrocene as a redox 10721


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to the underlying aryl primer nanocoating leads to a relatively smoother surface, as previously shown in Figure 4a,b.

reaction was conducted in the same way as reported above and the patterned sample analyzed by XPS and imaged by AFM. 3.6.1. XPS. Figure 7 shows a plot of the S/Au atomic ratio versus distance and, in the inset, Au 4f spectra taken at various

4. CONCLUSION In this work, 4-alkynylbenzenediazonium salt was electrochemically reduced on gold electrodes, therefore providing a platform grafted with 4-ethynylphenyl groups. The so-modified gold plates served for a photo-triggered thiol-yne click reaction with mercaptosilane. The thiol-yne reaction provided interfacial thiol ether linkages as testified by infrared spectroscopy (in the ATR mode). The Si−O−ethyl groups served for anchoring a sol−gel coating by in situ sol−gel synthesis using aminosilane and TEOS as a cross-linker. XPS was used to monitor the sequential chemical composition changes on the gold surface; the results show that the gold photoelectron peaks were gradually attenuated by the aryl, mercaptosilane, then by the sol−gel top layer. The modified layers were further studied by using blocking property measurement monitored by a ferrocene/ferrocinium (Fc/Fc+) redox couple. The existence of modified layers was confirmed by the observed reduction in the peak current density and increase in the charge transfer resistance. Furthermore, the thiol-yne reaction was conducted through Teflon masks to obtain patterned silanized aryl layers. Both XPS and AFM confirm that the aryl layers can be silanized locally by UV-induced thiol-yne reaction with mercaptosilane providing 12 nm thick stripes. This paper provides a unique UV-assisted strategy for bridging two coupling agents: an aryl layer from a diazonium salt precursor and a silane coupling agent. Interestingly, it is demonstrated that the photoinduced thiol-yne reaction can provide patterned surfaces with controlled size, composition, and thickness. This work shows another interesting side of the versatile aryl diazonium salt surface chemistry.

Figure 7. S/Au atomic ratio versus distance for patterned gold surface. (Inset) 3D display of Au 4f spectra taken at different electrode positions over the main axis. The electrode line scan was performed with a Thermo K Alpha machine.

successive positions of the gold plate with patterned silanized aryl layers. The spectra were taken with a step size of ∼100 μm and a spot size of 100 μm. It is clear that when the thiol-yne reaction is photo-induced through the windows cut in the mask, a silane layer is grafted to the underlying aryl primer nanocoating only at the site where the UV light could get through. The three waves built from the S/Au atomic ratio have a full width at half-maximum of about 800 μm, which is in line with the size of the windows cut from the Teflon mask. 3.6.2. AFM. A cross section of an AFM image of the electrode sample, at the limit zone separating the UV exposed and nonexposed areas, is displayed in Figure 8. The photo-induced steep height of ∼12 nm confirms that a thin layer was grafted only at the site where the UV light can penetrate the surface. Note that the thickness of the silane patterns determined by AFM is in line with that determined by XPS on continuous mercaptosilane layer (see XPS Analysis). Two statistical parameters have also been considered: the maximum surface peak height Sp and the maximum surface valley depth Sv. For the aryl primer nonexposed zone, we found Sp = 249 nm and Sv = −63 nm. After the photo-induced thiolyne reaction, Sp and Sv were found to be equal to 147 and −43 nm, respectively. This means that the grafting of a silane layer

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AUTHOR INFORMATION

Corresponding Authors

*(R.K.) E-mail: rafi[email protected]. *(M.M.C.) E-mail: [email protected] Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS All authors gratefully acknowledge Dr. Ahmed A. Mohamed (Delaware State University, Dover, DE, USA) for fruitful discussions and help proofreading the manuscript. A.K. thanks INRAP for the provision of travel funds to conduct research at ITODYS Laboratory. M.B.G. is grateful to the Tunisian Ministry of Higher Education and Research for Bourse

Figure 8. Cross section of AFM image of the patterned gold electrode sample. 10722


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d’Alternance scholarship. B.C. and M.M.C. acknowledge the French ANR for financial support through the POLARISafe project No. ANR-12-SECU-011-01. We acknowledge P. Decorse and A. Chevillot for their assistance with XPS and ATR analyses, respectively.

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Silanized Aryl Layers through Thiol-yne Photo-click Reaction

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